Deism is a belief that God created the universe, but left everything else to its own devices.

Some views differ in specifics. Many deists believe that the Big Bang was initiated by a god (of their choice), and that everything that happened since is the consequence of scientific laws "created" at the same time. In other cases, deism implies that said God set in motion all the events needed to make life on Earth inevitable — and thus is still responsible. Deistic beliefs tend to differ from usual religion by the fact that the creator god is either not worthy of worship, or worship is completely unnecessary. This gives deism a greater overlap with atheism and freethought compared to religion.

Deism arose and came to its most prominent during the philosophical change of the Enlightenment. As scientific knowledge increased, a climate of greater religious freedom developed in Europe and inconsistencies in the Bible were more freely discussed. A result of this process was an increasing belief that the universe followed naturalistic principles and God's active intervention (and by extension, traditional religious praxis like prayer, priests and scripture) was not necessary.

The classic text for deism is probably Thomas Paine's work The Age of Reason, where he states:

I believe in one God, and no more; and I hope for happiness beyond this life.

I believe in the equality of man; and I believe that religious duties consist in doing justice, loving mercy, and endeavouring to make our fellow-creatures happy.

But, lest it should be supposed that I believe many other things in addition to these, I shall, in the progress of this work, declare the things I do not believe, and my reasons for not believing them.

I do not believe in the creed professed by the Jewish Church, by the Roman Church, by the Greek Church, by the Turkish Church, by the Protestant Church, nor by any church that I know of. My own mind is my own church.

All national institutions of churches, whether Jewish, Christian or Turkish, appear to me no other than human inventions, set up to terrify and enslave mankind, and monopolize power and profit.

I do not mean by this declaration to condemn those who believe otherwise; they have the same right to their belief as I have to mine. But it is necessary to the happiness of man that he be mentally faithful to himself. Infidelity does not consist in believing, or in disbelieving; it consists in professing to believe what he does not believe.[1]

An analogy frequently used for the deist God is a watchmaker — the watchmaker assembles the pieces, and winds up the mechanism, but then just lets the watch run without interfering in it.

So, why did deists believe in a god at all? Partly because they were raised in a Christian milieu; partly because they thought of god as a natural focus and director of a seemingly rational, ordered universe. An argument from first cause thus seemed more plausible than it does in the light of modern science, which is now describing a universe that is more random and unpredictable than classical deism might have suspected.

The deist God is quite different to the God of traditional Abrahamic religions — it has no known personality, for example, and does not communicate with humans — and deists tend to see God as an abstract logical principle rather than an anthropomorphic being with wants and desires and a burning urge to control what we do with our pants off.

Unlike members of major revealed religions, many deists do not worship God, as they see no evidence that God (whatever form it takes) even wants to be worshiped. Deists who still wish to enjoy the trappings of church services will often attend Unitarian Universalist congregations, where lack of belief in the Bible is not generally stigmatized.

Deism holds a lot more common ground with atheism than it does with Abrahamic religions. Both hold the Bible and other religious works to be written mostly as human political works instead of being the holy word of any god. Both positions believe in naturalistic explanations for just about every phenomenon around them rather than trying to use "goddidit" to explain everything.

In fact, the only real difference lies in their explanation for the origins of time and space. Deists claim that a god created the universe and its rules, but did (and does) nothing else. Atheism simply takes this one step further by denying any existence, and therefore involvement of any god at all, in the beginnings of the universe or otherwise; in this way, deists can be thought of as atheists in everyday practicality.

Deism gradually declined due partly to the publication of Darwin's The Origin of Species, which finally gave a complete explanation for the diversity of life, and because it was always (and remains today) much more of a personal philosophy than an organized religious movement.

Deism still exists, but in a form distinct to the "classical" deism of the Enlightenment which was philosophically closer to present-day atheism.

The term "ceremonial deism" originated in a speech by Eugene Rostow in 1962, a dean of the Yale Law School. He used it in reference to exceptions carved out of the Establishment Clause of the United States Constitution for "government-sponsored religious speech that was 'so conventional and uncontroversial as to be constitutional.'"[2] At the time of his speech, the main exceptions were the addition of "One Nation Under God" to the The Pledge of Allegiance in 1954, and the creation of "In God We Trust" as the national motto in 1956, both during the height of the Red Scares.[2] Many dominionists conveniently forget that these are recent phenomena and instead like to think that the USA was founded as a Christian nation.

In 1984, the US Supreme Court decided that a "government-sponsored nativity scene that also included reindeer and candy canes was constitutional."[2] This was the first time that the Supreme Court used the term "ceremonial deism" in a decision.

“”I would suggest that such practices as the designation of "In God We Trust" as our national motto, or the references to God contained in the Pledge of Allegiance to the flag can best be understood, in Dean Rostow's apt phrase, as a form a "ceremonial deism," [Footnote 2/24] protected from Establishment Clause scrutiny chiefly because they have lost through rote repetition any significant religious content. See Marsh v. Chambers, 463 U.S. at 463 U. S. 818 (BRENNAN, J., dissenting).

Since the Supreme Court decision, Christians have tried to ceremonially-deism everything from the National Day of Prayer to Judge Roy Moore's erection of the Ten Commandments at a courthouse. The problem with this is that it's a double-edge sword: it subjects non-Christians to seeing Christian-sponsored trappings at every government office, while at the same time it dilutes Christianity symbolism to meaninglessness, lacking "any significant religious content".

Law professor Martha C. Nussbaum wrote in 2008 that "'Ceremonial Deism' is an odd name for a ritual affirmation that a Deist would be very reluctant to endorse, since Deists think of God as a rational causal principle but not as a personal judge and father."[4]